The sympathetic nervous system plays an important role in the regulation of metabolic and cardiovascular function. There is a well documented rise in sympathetic nerve activity to skeletal muscle (MSNA) at rest with aging in humans. The general experimental goal of the proposed studies will be to investigate one possible mechanism responsible for this age- related rise in tonic levels of MSNA. The specific aims are to determine: 1) if increases in total body fat (hyrodensitometry), total abdominal and abdominal visceral fat (computed tomography) with age are related to the increase in MSNA (microneurography); 2) if reductions in these same measures of body fat in older humans by caloric restriction are associated with decreases in MSNA; 3) if increases in these levels of body fat in young humans produced by over-feeding are associated with increases in MSNA; and 4) if changes in resting metabolic rate (RMR, open-circuit spirometry) are positively related to changes in MSNA. It is hypothesized that the measures of body fat will be directly related to MSNA and will contribute significantly to the age differences. Furthermore, changes in MSNA and RMR will be directly related to changes in total body, total abdominal, and abdominal visceral fat after the dietary interventions. The results of these studies will provide insight into one mechanism responsible for increased MSNA with age. This is important in light of the growing recognition of the role of the sympathetic nervous system in the physiology and pathophysiology of human aging.